A chip is said to "tombstone" when uneven melting of solder pulls the
chip from a horizontal to a vertical position. This set of Surface
Evolver datafiles models a rectangular chip wetted by liquid solder
at one end. The ultimate goal is to calculate the forces and torques
on the chip in an arbitrary position.
The datafiles start very simple and build in complexity.
The list of datafiles is given below, with brief descriptions.
Each datafile has its own page, elaborating on added features.
The complete set of datafiles can be downloaded as
tombdata.zip (DOS format) or
tombdata.tar (UNIX format).

NOTE:These models require
Evolver version 2.11 (March 1, 1999)
or higher to run.
One of the benefits of writing my own software is that if it needs
features, I can add them. Some of the datafiles could run on old
versions with a little editing, but some definitely need the new
features.

The chip has a nonwettable region (pink) and a wettable region (red).
The liquid solder surface is yellow. The rectangular solder pad on
the circuit board is green. Physical parameters that will need to be
specified in the datafiles are the chip and pad dimensions, chip and
pad contact angles for the solder, the density of the solder, and the
surface tension of the liquid solder. All of these need to be given
in one consistent system of units, such as cgs or MKS. We will use cgs
here.

The general sequence of steps in modelling something like this is

Create a physical/mathematical model, in the sense of
deciding on geometry, energies, and constraints.

Write scripts for the calculation of whatever forces, torques, etc.
you want as output data.

Estimate the accuracy of your data as a function of refinement,
using known special cases or by looking at the convergence of a
sequence of values for successive refinements.

Write scripts to automatically evolve and collect data over a range
of parameters.

There are a number of general themes in the sequence of models:

The ease of setting up a model where the solder-chip contact facets
are explicitly represented as facets, versus the ease of evolution when
those facets are omitted and their effects accounted for by edge integrals.
The single-letter datafiles below have contact facets, and the double-letter
ones do not.

Certain defaults have to be changed because the surface is
physically so small.

The Datafiles:

The sequence of datafiles starts simple and adds features. The sequence
is not strictly linear; the main sequence keeps the contact surface on
the chip explicitly represented by facets, and a sideline replaces those
facets with edge integrals. Each datafile link here opens a commentary
page, from which one can also open the datafile in a separate browser
window. The idea is to be able to look at the commentary and datafile
simultaneously. The datafiles themselves also contain a little commentary.
You are also urged to have the Evolver running and run the models
as they are discussed.